The General Electric GAU-8/A Avenger is a 30 mm, hydraulically-driven seven-barrel Gatling-type rotary cannon that is mounted on the United States Air Force's A-10 Thunderbolt II. It is among the largest, heaviest and most powerful aircraft cannon in the United States military, and when the entire cannon with its drum-style ammunition magazine is considered, is the single largest forward-firing cannon ever fitted into an aircraft.[citation needed] Designed specifically for the anti-tank role, the Avenger delivers a very powerful round at a high rate of fire.

Contents

History

The GAU-8 was created as a parallel program with the A-X (or Attack Experimental) competition that produced the A-10. The specification for the cannon was laid out in 1970,[2] with General Electric and Philco-Ford offering competing designs. Both of the A-X prototypes, the YA-10 and the Northrop YA-9, were designed to incorporate the weapon, although it was not available during the initial competition, and the M61 Vulcan was used as a temporary replacement. Once completed, the entire GAU-8 assembly (correctly referred to as the A/A 49E-6 Gun System)[3] represents about 16% of the A-10 aircraft's unladen weight.

The gun is placed slightly off center in the nose of the plane with the front landing gear positioned to the right of the center line, so that the actively firing cannon barrel is directly on the aircraft's center line. The Russian Gryazev-Shipunov GSh-6-30 is a similar class of weapon, although it is lighter with a higher fire rate, but has a lower muzzle velocity and overheats more quickly.

The gun is loaded using Syn-Tech's linked tube carrier GFU-8/E 30 mm Ammunition Loading Assembly cart. This vehicle is unique to the A-10 and the GAU-8.[4]

Design

The GAU-8 itself weighs 620 pounds (280 kg), but the complete weapon, with feed system and drum, weighs 4,029 pounds (1,828 kg) with a maximum ammunition load. It measures 19 ft 5½ in (5.931 m) from the muzzle to the rearmost point of the ammunition system, and the ammunition drum alone is 34.5 inches (88 cm) in diameter and 71.5 inches (1.82 m) long.[5] The magazine can hold 1,174 rounds, although 1,150 is the more normal load-out. Muzzle velocity when firing Armor-Piercing Incendiary rounds is 3,250 feet per second (990 m/s), almost the same as the substantially lighter M61 Vulcan's 20 mm round.

GAU-8 inside A-10.

The standard ammunition mixture for anti-armor use is a four-to-one mix of PGU-14/B Armor Piercing Incendiary, with a projectile weight of about 15.0 oz (425 grams or 6,560 grains) and PGU-13/B High Explosive Incendiary (HEI) rounds, with a projectile weight of about 12.7 oz (360 grams). The PGU-14/B's projectile incorporates a lightweight aluminum body, cast around a smaller caliber depleted uranium penetrating core.[6] The Avenger is lethal against tanks and all other armored vehicles.

A very important innovation in the design of the GAU-8/A shells is the use of aluminum alloy cases in place of the traditional steel or brass.[7] This alone adds 30% to ammunition capacity for a given weight. The shells also have plastic driving bands to improve barrel life. They are imposing to examine and handle, measuring 11.4 inches (290 mm) in length and weighing 1.53 pounds (0.69 kg) or more.[5][7]

The Avenger's rate of fire was originally selectable, 2,100 rounds per minute (rpm) in the low setting, or 4,200 rpm in the high setting.[8] Later this was changed to a fixed rate of 3,900 rpm.[9] In practice, the cannon is limited to one and two-second bursts to avoid overheating and conserve ammunition; barrel life is also a factor, since the USAF has specified a minimum life of at least 20,000 rounds for each set of barrels.[10] There is no technical limitation on the duration the gun may be continuously fired, and a pilot could potentially expend the entire ammunition load in a single burst with no damage or ill effects to the weapons system itself. However, this constant rate of fire would shorten the barrel life considerably and require added barrel inspections and result in shorter intervals between replacement.

Each barrel is a very simple non-automatic design having its own breech and bolt. Like the original Gatling gun, the entire firing cycle is actuated by cams and powered by the rotation of the barrels.[7] The barrels themselves are driven by the aircraft's dual hydraulic system.[10]

The GAU-8/A ammunition is linkless, reducing weight and avoiding a great deal of potential for jamming. The feed system is double-ended, allowing the spent casings to be recycled back into the ammunition drum,[11] instead of ejected from the aircraft, which would require considerable force to eliminate potential airframe damage. The feed system is based on that developed for later M61 installations, but uses more advanced design techniques and materials throughout, to save weight.[5]

Variants

Some of the GAU-8/A technology has been transferred into the smaller 25 mm GAU-12/U Equalizer developed for the AV-8B Harrier II aircraft, which is about the same size as the M61 but is considerably more lethal. GE has also developed the GAU-13/A, a four-barreled weapon using GAU-8/A components, which has been tested in podded form as the GPU-5/A, and the Avenger forms the basis for the Dutch-developed Goalkeeper naval air-defence gun. No current or contemplated aircraft other than the A-10, however, carries the full-up Avenger system.[5]

Firing system

The GAU-8/A Avenger Gatling gun next to a VW Type 1. Removing an installed GAU-8 from an A-10 requires first installing a jack under the aircraft's tail as the cannon composes the majority of the aircraft's forward weight.

Accuracy

The GAU-8/A is not only extremely accurate, it can fire from 2100 to 4200 shells per minute without complications. The 30-mm shell has twice the range, half the time to target, and three times the mass of projectiles carried by comparable Close Air Support aircraft.[1]

The muzzle velocity of the GAU-8/A is about the same as that of the M61 Vulcan cannon, but its improved ammunition is more destructive and has superior ballistic properties. Its time of flight to 4,000 feet (1,200 m) is 30 percent less than that of an M61 round, the projectile decelerates much less rapidly after leaving the barrel, and it drops a negligible amount, about 10 feet (3.0 m) over the distance. The GAU-8/A accuracy when installed in the A-10 is rated at "5mil, 80 percent", meaning that 80 percent of rounds fired at 4,000 feet (1,200 m) will hit the target within a 20 feet (6.1 m) radius circle. By comparison, the M61 is rated at 8 milliradians.[7][12]

Recoil

Each barrel fires when it reaches roughly the 9 o'clock position, when viewed from the front of the plane. Because the gun's recoil forces could push the entire plane off target during firing, the weapon itself is mounted off-center in the other direction, toward the 3 o'clock position, so that the firing barrel lies directly on the aircraft's center line. The firing barrel also lies just below the aircraft's center of gravity, being bore sighted along a line 2 degrees below the aircraft's line of flight. This arrangement accurately centers the recoil forces, preventing changes in pitch and/or yaw when fired. This configuration also provides space for the front landing gear, which are mounted slight off-center on the right side of the nose.[3]

The GAU-8/A utilizes recoil adapters. They are the interface between the gun housing and the gun mount. By absorbing (in compression) the recoil forces, they spread the time of the recoil impulse and counter recoil energy transmitted to the supporting structure when the gun is fired.

The A-10 engines were initially susceptible to flameout when subjected to gases generated in the firing of the gun. When the GAU-8 is being fired, the smoke from the gun can make the engines stop, and this did occur during initial flight testing.[2] Gun exhaust is essentially oxygen-free, and is certainly capable of causing flame-outs of gas turbines. The A-10 engines have a self sustaining combustion section. When the gun is fired the igniters come on to ensure no flame out occurs.[13]

The recoil force of the GAU-8/A[14] is 10,000 pounds-force (45 kN),[3] which is slightly more than the output of one of the A-10's TF-34 two engines (9,065 lbf / 40.3 kN each).[15] While this recoil force is significant, in practice cannon fire only slows the aircraft a few miles per hour.[13]